Torque-transmitting devices such as clutches or brakes are heavily used throughout the automotive industry. For example, vehicle transmissions employ a multitude of clutches to engage and disengage the gearsets of the transmission to provide forward and reverse gear ratios. The clutch includes a reaction plate for exerting a compression force. The clutch also includes a friction plate disposed adjacent the reaction plate for frictionally engaging the reaction plate to transfer a driving torque between the reaction plate and the friction plate when the compression force is applied. The clutch further includes a friction material layer adhered to the friction plate that opposes the reaction plate. The friction material layer is configured to be compressible by the reaction plate. The clutch further includes a fluid lubricant disposed between the reaction plate and the friction plate for providing a lubrication layer between the plates during clutch engagement.
Three dominant lubrication states exist during clutch engagement: hydrodynamic (HD), soft elasto-hydrodynamic (soft-EHL) and boundary. Hydrodynamic lubrication state is characterized by a thick fluid film thickness, low nominal contact pressure of the plates of the clutch, and an extremely low coefficient of traction/friction. Thus, torque transfer during in the hydrodynamic lubrication state is difficult to achieve.
During the soft elasto-hydrodynamic lubrication state, the sharp groove edges and/or rough fibers topography of the friction material operates to break through the hydrodynamic film and create islands of film separating the mating surfaces of the clutch plates. The islands are unevenly distributed over the surface area due to the non-homogenous nature of friction material, waviness of the plates and uneven load distribution. Since the nominal load on the friction plate is constant, the islands of fluid experience much greater pressure than exists in the hydrodynamic state. As the lubrication fluid becomes thinner its shear rate rapidly increases, which allows sufficient torque to be transferred through the fluid.
During the boundary lubrication state the soft-EHL fluid experiences break down due to a speed decrease and temperature rise. Further, the active additives in the surface of the friction material layer are activated and create a low-shear protective layer of fluid, which acts like a solid lubricant which interacts with the friction material layer to create a solid to solid coefficient of friction which is closer to the EHL value. A tribo-chemical layer is resultant of chemical reaction between the fluid lubricant additives and mating surfaces of the clutch plates. When the fluid lubricant deteriorates, the tribo-chemical layer undergoes shearing. The chemical bonds break-down, additives in the fluid are depleted. The tribo-chemical layer is replenished by using new non-depleted additives from the fluid. As additive concentration in the fluid decrease below a predefined critical level, the local coefficient of friction increases in magnitude and leads to uneven clutch engagement (shudder or stick/slip) or further temperature increase. Provided the temperature increases, the high temperature may result in damage to the friction material (wear, tearing, and glazing). The reaction plate may also be damaged which may include hot spotting.
The soft-EHL is the optimum state for clutch operation for the following reasons: (1) The thin fluid film traction provides friction coefficients sufficient to transfer the required torque. (2) The thin film traction does not generate excessive heat, which may damage the friction material (wear, tearing and glazing) and reaction plate (hot spotting) leading to the loss of friction and resulting in shudder. (3) The surface active friction modifiers are not consumed during the soft-EHL, so fluid deterioration is delayed. As long as fluid base stock is chosen from high level grades, the bulk oxidation is not primary failure mode of the clutch up to a fluid sump temperature of 135 degrees Celsius. (4) Fluid film as well as friction material have damping capabilities that reduce the amplitude of friction interface self-induced and/or external vibration. (5) The traction/friction coefficient is proportional to the viscous drag, which decreases with increasing temperature (this is typical for the clutch engagement cycle) positive friction/slip slope. (6) Compared to other lubrication regimes, the clutch can operate in the soft-EHL mode for sufficiently long periods of time without any damage or failure of the mating surface.
There is a need in the art to provide a new and improved clutch that prolongs the soft EHL state during clutch engagement. The new and improved clutch should maintain a sufficient coefficient of friction and not create excessive heat that will damage the friction material layer of the clutch.